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Abstract

The prospects for cost-effective flat plate (non-concentrator) solar cells based on III-V compound semiconductors (e.g., GaAs, InP, AlAs, and their alloys) are reviewed. Solar cells made in III-V materials are expensive, but outperform solar cells in every other materials system. The relatively high cost of compound semiconductor wafers necessitates a means to eliminate their use as substrates for epitaial growth of conventional III-V solar cells. There are several approaches to this end, including thin-film solar cells on low-cost, dissimilar substrates such as glass, ceramics, and metal sheets; III-V solar cells epitaxially grown on silicon wafers; film transfer (‘epitaxial lift off’) techniques that allow re-use of the seeding substrate; and assembled arrays of small III-V solar cells on low-cost substrates. Grain boundary effects in polycrystalline III-V films can severely degrade solar cell performance, and impede the application of established thin-film technologies, as developed for amorphous silicon and II-VI semiconductor photovoltaics, to III-V semiconductor-based solar cells. The nearly fifty years of effort in developing thin-film III-V solar cells has underscored the difficulty of achieving large-grain sizes and/or low recombination grain boundaries in polycrystalline films of III-V semiconductors.

Background

The main attraction of III-V solar cells is their superior performance. III-V solar cells in the form of sophisticated epitaxial multi-junction device structures have demonstrated the highest conversion efficiencies (~40%) of any photovoltaic technology to date. The relevant performance data of various III-V solar cell technologies are summarized in Table 1, along with silicon solar cells that provide a performance benchmark. Secondary advantages of III-V solar cells relative to silicon solar cells include higher radiation resistance (crucial for space power, but not an over-riding consideration for terrestrial photovoltaics); more tolerance of increased operating temperatures, and higher operating voltages which reduce series resistance losses. It should be kept in mind that the best commercial silicon solar cells are already at ~20% efficiency (e.g., Sanyo HIT crystalline/amorphous silicon solar cell (Taguchi et al. 2005), and therefore, the areal cost of alternative single-junction III-V solar cells with similar efficiencies—and without the leveraging effect of optical concentration—will have to be comparable to that of the current premium single-crystal silicon solar cell technology. By the same reckoning, it can be inferred as a corollary that III-V solar cells that cost more than silicon on an areal basis will need to have a substantially higher conversion efficiency than the best silicon solar cells in order to challenge the predominance of silicon photovoltaics.